Backward-wave tube with periodic electrostatic focusing



Dec. 15, 1970 ALEXEENKO ETAL 3,548,247

BACKWARD-WAVE TUBE WITH PERIODIC ELECTROSTATIC POCUSING Filed Feb. 29, 1968 United States Patent US. Cl. 315-3.6 3 Claims ABSTRACT OF THE DISCLOSURE A backward-Wave tube with periodic electrostatic focusing, in which a finned slow-wave system of an interdigitated type is used to attain a high level of output power within a wide electrical frequency tuning range, the fins of the system having respective apertures which cooperatively define axial channels to provide passage for the electron beam.

The invention relates to devices for generating and amplifying super-high frequency and, in particular, to backward-wave tubes with periodic electrostatic focusing.

Conventional backward-wave tubes with periodic electrostatic focusing are known to utilize a slow-wave structure of the inter-digitated fin type. Each fin of said structure is provided with one aperture, the orthogonal projections of all the apertures coinciding with one another so that said apertures form a channel wherethrough the electron beam is free to pass. A periodic electrostatic focusing is attained by virtue of different potentials with respect to the cathode, applied to the adjacent fins.

A disadvantage inherent in said structure resides in the fact that high current circuit in the systems with periodic electrostatic focusing within a wide range of variation of focusing voltage is obtainable at low perveance values of the electron beam involved.

On the other hand, to obtain a sufiiciently high level of the output power, fairly high working current values in the system channel are required. Therefore, despite the fact that backward-wave tubes feature a comparatively high output power (from 0.1 to 1.0 w.), they require a perveance approximating its limiting value at which beam focusing is still obtainable.

In this case, while retuning the frequency by varying the average voltage of the beam, the adjustment of focusing voltage is necessary to be carried out at every particular point of the range, i.e. the frequency retuning of the generated oscillations will be accomplished by voltage variation of two supply sources. If frequency retuning of backward-wave tube oscillations is performed by varying the potential of only one supply source, the range of electrical retuning will not exceed to Approximation of the beam perveance to its limiting value likewise impairs the output power, since insignificant fluctuations of the beam will adversely affect the current passage and, consequently, will result in dropping of the output power level.

A primary object of the present invention is to eliminate the abovementioned disadvantages and to provide a backward-wave tube with electrostatic focusing, wherein frequency retuning will be possible within a wider range of the frequency spectrum and be effected by voltage variation of one supply source only.

A specific object of the present invention is to impart such a shape to the fins as to widen the retuning range of frequencies generated by the tube involved.

This object is accomplished due to the fact that a plu- "ice rality of apertures are made in each fin of the structure in such a way that they form axial ducts for the passage of the electron beam.

In this case the perveance of each electron beam may be substantially reduced, the current passing through the slow-wave structure remaining at a reasonably high value.

It is expedient that the apertures in one of the fins be so made that its contours coincide in the orthogonal projection with the contours of similar apertures of all the other fins of the structure.

The apertures in the slow-wave structure may be of slitlike shape.

An embodiment of the present invention will be described hereinbelow by way of illustration with reference to the accompanying drawings, wherein:

FIG. 1 is a section view through a backward-wave tube of the invention; and

FIG. 2 is a section taken along line 1III of FIG. 1.

The backward-wave tube with electrostatic focusing comprises; a slow-wave structure 1 of the interdigitated fin type, an electron gun 2, a collector 3, a waveguide 4, a final absorber 5, and a vacuum-tight shell 6.

Each fin 7 (FIG. 2) of the slow-wave structure 1 is provided with a plurality of slit-shaped apertures 8. The fins 7 of the tube (FIG. 1) are so arranged that the apertures 8 of one of the fins 7 are disposed coaxially with those of the other fins to form a plurality of ducts for passing the electron beam produced by the electron gun 2. The electron gun 2 comprises an end-type cathode 9 and a focusing electrode 10 which is essentially a plate with apertures similar in shape to the apertures 8 in the fins 7 of the slow-wave structure. The focusing electrode 10 whose potential is equal to or less than that of the cathode 9, is intended for spatial separation of the electron beam into several bunches.

All the fins of the slow-wave structure are combined in two groups which are electrically insulated from each other by means of ceramic insulators 11 so that the adjacent fins are insulated from one another.

The above groups of fins form two combs 12 and 12' (FIG. 1) fitting into one another. One of the combs, 12, is electrically connected to the metallic vacuum-tight shell 6 of the tbue and the other comb 12 is secured relative to the comb 12 and the vacuum-tight shell 6 on supports 15 using ceramic insulators 11.

A differential potential i created between the combs 12 and 12' and, consequently, between the adjacent fins, by means of a potential source so that a periodic electrostatic field is formed along the slow-wave system, thereby making possible the focusing of electron beams.

Two high-frequency chokes are provided to ensure a high-frequency contact between the comb 12 isolated from the vacuum-tight shell and the Waveguide 4, as well as with a sector of two-wire line 13, wherein the final absorber 5 is placed.

The apertures 8 in the fins of the slow-wave structure 1 may be diversely shaped, so as to be circular, square, slit-like, cruciform-type, and the like.

Also possible are cases where it is advantageous to interconnect the apertures in each fin by auxiliary slits.

Experience has established that apertures of slit-shaped type are the most effective.

The sizes of the apertures 8 are so selected that the coupling impedance at the most removed points from the edge of the aperture, substantially differs from zero. In this case the electron beam will always be within the zone of an intense high-frequency field.

The number of apertures and, consequently, the width of the fins are dependent upon the required current value of the electron beam at the preset potentials applied to the structure fins and upon the perveance.

As there is theoretically no limit for increasing the width of the fins in said structure, it is possible to develop tubes which will feature high current values and, consequently, a high level of output power.

An electron beam in the form of several spatially separated rays is produced by the electron gun 2. The first fin of the slow-wave structure conductively coupled to the vacuum-tight shell 6, serves as the gun anode. Frequency retuning of the oscillations generated by the backwardwave tube is accomplished by varying the potential of the comb 12' insulated from the vacuum-tight shell. Concurrently, said potential is used for the electron beam focusing.

The employment of the slow-wave structures, which incorporate several ducts for the electron beam to pass, provides for the possibility of developing devices with a wavelength band from 8 mm. to 60 cm. The range of electrical retuning of the short-Wave portion of a microwave band amounts to 20 to 40%, whereas that of a decimetric band, is nearly equal to an octave. As to the level of the output power, the backward-wave tube with electrostatic focusing is on a par with magnetically focused backward-wave tubes, the former featuring much less weight and size.

For example, the output power of devices with a 3-cm.

hand is within 100 to 150 milliwatts, the input power being from 10 to 15 watts, whereas the retuning range is from to Devices with the output power from 1 to 2 w. are likewise available.

What is claimed is:

1. A backward-wave tube with periodic electrostatic focusing, said tube comprising a slow-wave system which includes two electrically insulated combs with successively interdigitated fins, and means for applying different focusing potentials to the combs, each of said fins having a plurality of random apertures which cooperatively define axial channels for the passage of the electron beam.

2. A tube as claimed in claim 1, wherein said apertures in said fins are of the same size and shape and are aligned axially.

3. A tube as claimed in claim 2, wherein said apertures are rectangular slits.

References Cited UNITED STATES PATENTS 2,810,854 10/ 1957 Cutler 3153.6 3,002,123 9/1961 Peter 31539.3X 3,018,448 1/ 1962 Warnecke 315--3.6X 3,114,072 12/ 1963 Belohoubek 3153.5X 3,353,057 11/1967 Kato et a1. 315-3.5 3,400,297 9/ 1968 Miyamoto 3153.5

HERMAN KARL SAALBACH, Primary Examiner M. NUSSBAUM, Assistant Examiner US. Cl. X.R. 

